This invention relates to a self-triggering imaging device for imaging radiation, in particular, but not exclusively, to an imaging device for a self-triggerable imaging system; the imaging device comprising an array of image elements.
Imaging devices comprising an array of image elements of various types are known.
Charged coupled image sensors (also known as charged coupled devices (CCDs)) form one type of known imaging device. A CCD type device operates in the following way:
1. Charge is accumulated within a depletion region created by an applied voltage. For each pixel (image cell) the depletion region has a potential well shape and constrains electrons under an electrode gate to remain within the semiconductor substrate.
2. Voltage is applied as a pulse to the electrode gates of the CCD device to clock each charge package to an adjacent pixel cell. The charge remains inside the semiconductor substrate and is clocked through, pixel by pixel, to a common output.
During this process, additional charge cannot be accumulated.
Another type of imaging device which is known is a semiconductor pixel detector which comprises a semiconductor substrate with electrodes which apply depletion voltage to each pixel position and define a charge collection volume. Typically, simple buffer circuits read out the electric signals when a photon is photo-absorbed or when ionising radiation crosses the depletion zone of the substrate. Accordingly pixel detectors of this type typically operate in a pulse mode, the numbers of hits being accumulated externally to the imaging device. The buffer circuits can either be on the same substrate (EP-A-0,287,197) as the charge collection volumes, or on a separate substrate (EP-A-0,571,135) that is mechanically bonded to a substrate having the charge collection volumes in accordance with, for example, the well known bump-bonding technique.
A further type of device is described in International application WO95/33332. In WO95/33332, an Active-pixel Semiconductor Imaging Device (ASID) is described. The ASID comprises an array of pixel cells including a semiconductor substrate having an array of pixel detectors and a further array of pixel circuits. The pixel detectors generate charge in response to instant radiation. Each pixel circuit is associated with a respective pixel detector and accumulates charge resulting from radiation incident on the pixel detector. The pixel circuits are individually addressable and comprise circuitry which enables charge to be accumulated from a plurality of successive radiation hits on the respective pixel detectors. The device operates by accumulating charge on the gate, for example, of a transistor. Accordingly, analogue storage of the charge value is obtained. At a determined time, the charge from the pixel circuits can be read out and used to generate an image based on the analogue charge values stored in each of the pixel circuits.
CCD devices suffer from disadvantages of limited dynamic range, due to the limited capacity of the potential well inside the semiconductor substrate, and also to the inactive times during which an image is read out. Pulse counting semiconductive pixel devices also have the disadvantage of limited dynamic range. As these devices read the pixel contact when a hit is detected, they suffer from saturation problems at high counting rates. The semiconductor pixel device according to WO95/33332 provides significant advantages over the earlier prior art by providing a large dynamic range for the accumulation of images.
It has been proposed to utilise the above-mentioned CCD and semiconductor devices to replace the film used in conventional radiation imaging systems, in order to provide real-time imaging and a more controlled lower dosage of radiation for a given exposure.
In a known arrangement, a CCD is electrically connected to an X-ray source. When the X-ray source is energised a start signal is transmitted along the connecting wire to the CCD and its control circuitry to begin image acquisition and read-out.
In a optional arrangement disclosed in U.S. Pat. No. 5,513,252 there is no connection to the X-ray source. Instead, the CCD is continually read-out prior to radiation. A signal derived from the CCD is compared with a reference level. If the signal exceeds the reference level, the image acquisition of the CCD is initiated, that is to say the CCD stops being read out and the image starts to accumulate on the CCD.
European Patent Application Publication No. 0 756 416 A1 discloses a CCD used as an imaging device in which charge accumulated in the CCD elements is clocked from several rows into a register in order to sum the charges. The summed result is put to a threshold test. Onset of X-ray radiation is detected when the signal applied to the threshold test exceeds a reference level. Image acquisition is then initiated, as described above i.e. only then will the CCD start accumulating the image.
In yet another arrangement the X-ray source and CCD have again no physical connection. A further sensor is arranged close to the imaging array for the CCD to detect the onset of X-ray radiation. On detection of incident X-ray energy, the sensor sends a signal to the CCD control circuitry to initiate image acquisition, as before.
The foregoing prior art systems involve a delay between activation of the radiation source and initiation of image acquisition. Since in radiation imaging, in particular X-ray imaging, radiation devices should be kept as low as possible it is desirable to reduce the delay as much as possible.
In accordance with an embodiment according to a first aspect of the invention there is provided a semiconductor radiation imaging device including:
an array of image elements, each image element of said array comprising an image element detector cell which generates charge in response to radiation incident on said image element and image element circuitry for accumulating said charge from said image element detector cell and for selectively outputting a signal representative of accumulated charge; and
a radiation sensor element, said radiation sensor element comprising a radiation detector cell integral with said image element detector cells for generating charge in response to incident radiation on said radiation sensor element and a radiation sensor output for continuously supplying charge from said radiation detector cell.
Preferably, said radiation sensor output is adapted for continuously supplying a signal representative of charge generated in response to radiation incident on said radiation sensor element.
In a preferred embodiment said radiation detector cell is more responsive to incident radiation than an image element detector cell.
Suitably, said radiation detector cell comprises a larger area than said image element detector cell.
Advantageously, said radiation detector cell comprises a guard ring for said array of image element detector cells and optionally the guard ring is substantially continuous.
The imaging device includes a unitary substrate comprising said image element detector cell, said image element circuitry and said radiation detector cell.
Alternatively, the imaging device includes a first and second substrate, the first substrate comprising said image element detector cell and said radiation detector cell, and the second substrate comprising said image element circuitry.
An embodiment in accordance with a second aspect of the invention comprises a self-triggerable imaging system including an imaging device as described in the foregoing paragraphs, the imaging system further comprising control circuitry for controlling the imaging device, an image processor for processing charge values from said array of image elements to form an image on a display device, and threshold circuitry coupled to said radiation sensor output.
Typically, the imaging device comprises threshold circuitry coupled to the radiation sensor output.
In a preferred embodiment the threshold circuitry is operable to output a trigger signal to the control circuitry for the charge or signal on the radiation sensor output exceeding a threshold value.
Preferably, the control circuitry is configurable to initiate image acquisition responsive to said trigger signal from the threshold circuitry. Optionally, the control circuitry is configurable to periodically monitor for the trigger signal.
Suitably, the imaging system further comprises an analogue to digital converter for converting an analogue signal supplied on the radiation sensor output to a digital value, and the digital value is subjected to a threshold test.
In an embodiment according to a third aspect of the invention, there is provided a method comprising providing an imaging device as described above, and including the steps of monitoring said radiation sense output, comparing signals from said radiation sense output with a threshold and outputting a control signal for initiating reading of the array for said charge or signal exceeding the threshold.
In accordance with an embodiment of a yet further aspect of the invention, there is provided a method for providing a trigger signal for radiation imaging comprising, providing a radiation sensor integral with an image plane including an array of radiation detectors for collecting incident radiation for forming a radiation image, the radiation sensor being more responsive to incident radiation than a one of the array of radiation detectors and generating a signal indicative of radiation incident on the radiation sensor, comparing the signal with a threshold, and outputting a control signal for initiating reading of the array for the signal exceeding the threshold.
In a preferred embodiment the method comprises further providing control circuitry for controlling reading the array responsive to said control signal.
Optionally, the step of monitoring comprises periodically monitoring the radiation sensor output or for the control signal.